Convergent evolution is an absolutely amazing phenomenon. Why do creatures, both closely and distantly related, sometimes develop the same body types or adaptations? Saber-teeth are my most favorite convergent character, but I recently found out about another case of convergence that is just as interesting. In South America there used to be a group of animals called Litopterns, the most famous members of the group looking something like an odd-toed camel with a fleshy schnoz hanging off the front of their faces. The smallest representative of the group was called Thoatherium, and it looked surprisingly like a horse.
According to G.G. Simpson in his book Splendid Isolation, Thoatherium was "even more completely one-toed than one-toed horses, including all living members of the horse family." This is even more significant when it is realized that Thoatherium is of Miocene (23.03 to 5.33 mya) age, while actual equids had not fully lost their side toes until the Pliocene (5.33 million to 1.81 million years ago). Unlike derived horses, though, Thoatherium was a browser and not a grazer, the one-toed horses being adapted to life in grasslands while Thoatherium probably chewed on vegetation that hung higher off the ground.
I wish I could continue to discuss the anatomy of habits of Thoatherium, but whatever information might be available about it is either hidden or out of my reach. It is very interesting how Thoatherium evolved a form like that of a 2-foot-long horse millions of years before true horses were running on their single digits alone, and I think that the case of this South American ungulate deserves some further investigation.
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Agreed; I'd love to see more information on Thoatherium. It's perhaps my favorite South American critter, even more so than Machrauchenia or Thylacosmilus.
A big mystery to me is why its lineage became extinct long before the land bridge really opened up. I'd think an animal so highly evolved for competent running would have an advantage over its clumsier litoptern relatives that outlived it, but apparently not.
Obviously, then, horses are Thoatherium mimics.
Homalodotheres are also fascinating in the same way - the South American answer to chalicotheres!
To quote Spock: "Fascinating."
Wow.
Not that I have anything of incredible intellectual value to add. But this is why I read you, Brian: I learn new stuff, and you present things with a sense of wonder that makes it all exciting. Thanks again!
"A big mystery to me is why its lineage became extinct long before the land bridge really opened up. I'd think an animal so highly evolved for competent running would have an advantage over its clumsier litoptern relatives that outlived it, but apparently not."
There was a proterotheriid (the group to which Thoatherium belonged) that survived into the early Holocene (Neolicaphrium recens), but its toes were not quite as reduced as those of Thoatherium. There wasn't really any competition with the macraucheniids, which must have occupied a quite different niche.
By the way, some hyracoids (which were much bigger than today's hyraxes) also had reduced side toes, although not to the extent as in Thoatherium and horses.
Well, I had heard of thoats before, but I thought they were all on Barsoom.
I have a long post (or two) planned on litopterns - should appear some time within the next couple of months. They're one of those frustrating groups where there are tens of taxa yet we only ever hear about the same two or three.